Nondestructive testing of articles and materials has become an integral part of quality control in modern manufacturing industries. This type of testing enables on-line and intensive evaluation of the structural soundness of products. One of the most commonly used forms of nondestructive testing is radiographic images taken on industrial materials. Industrial X-rays have been used for many years in the testing of support beams used in the construction of buildings, bridges and the like. They are particularly useful in the evaluation of welds and in testing metal plates for minute flaws which could affect performance.
As industrial demands on materials become more stringent and the tolerance for flaws becomes reduced, more precise testing methods are required. In all imaging processes, including photography and radiography, there is an inherent limit in the resolution available through the process because of the physical elements used. In the practice of modern industrial X-ray procedures, the use of intensifying screens adds a further limit on the resolution available in radiographs. It has heretofore been generally accepted that the phosphor grains in intensifying screens and the screens themselves were the limiting factor in the graininess or resolution available in radiographs used in nondestructive testing (cf. Nondestructive Testing, 2d Ed. Warren J. McGonnagle, Science Publishers, 1971, pages 119-123, Radiography in Modern Industry, 3d Ed., Eastman Kodak, 1969, pages 34-38, and Physics of Industrial Radiology, R. Halmshaw, London, Heywood Books, 1966, pp. 110 and 176). This limitation was believed to be a result of the fact that visible radiation emitted from the phosphor grain is spread out rather than projected in a linear path like the incident X-rays.
Radiographic emulsions used in industrial screen/film X-ray procedures typically have emulsions where the average grain size is above 0.5 micrometers (e.g., U.S. Pat. No. 3,922,545, col. 13, lines 25-46) and generally over 1 micrometer (e.g., U.S. Pat. No. 3,753,714, col. 4, lines 34-40). U.S. Pat. Nos. 4,177,071 and 4,130,428 discloses a range of 0.25 to 1.2 micrometers for the grain size, but the examples are only of emulsions having average grain sizes of 0.5, 0.6, 0.7 and 0.8 micrometers.
Furthermore, these patents show color photographic emulsions, wide distribution of grain size, high coating weights of silver halide, low contrast values, and low optical densities.